Electrical contacting surfaces



March 17, 1964 M. ARNOLD ELECTRICAL CONTACTING SURFACES Filed Oct. 31, 1961 A 1m IIH INVENTOR By S. M. ARNOLD ATTORNEY United States Patent 3,125,654 ELECTRICAL CONTACTING SURFACES Sydney M. Arnold, Chatham, N.J., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y.,. a corporation of New York Filed Got. 31, 1961, Ser. No. 149,0% 6 Claims. (til. 200-166) This invention relates to electrical components and more particularly to electrical contacts used to make and break circuits.

bismuth intermediate the substrate and final tin-containing surface layer avoids whisker growth in such components. The invention will be more readily understood from the following more detailed description, taken in conjunction with the accompanying drawing in which:

FIG. 1 is a front elevational view of an apparatus suitable for use in producing a film of metal by vacuum evaporation in accordance with the present invention, and 1 FIG. 2 is a front elevational view, partly in section, of a switching element including contacting surfaces in accordance with this invention.

Referring more particularly to FIG. 1, there is shown an apparatus suitable for depositing metal films by vacuum evaporation techniques. The figure depicts a vacuum chamber 11 containing filament 12 and platform 13 which latter is employed as a positioning support for substrate 14. Mask 15 is utilized as shown to restrict the deposition of the film to the desired area. The ends of filament 12 are connected to electrical leads 16 to permit flow of current therethrough from a source not shown.

In FIG. 2 there is shown a practical application of the novel structure described herein. The figure shows a typical relay spring having deposited thereon electrical contacts comprising copper and tin. Shown attached to relay springs 21 are contacts 22 which consist essentially of outer tin layer 23 and an inner copper layer 24, each layer having been deposited by vacuum evaporation techniques.

As disclosed, the present invention contemplates the use of a substrate upon which the contacting surface is to be deposited. To be suitable, the substrate should he possessed of a smooth surface which is completely free from sharp changes in contours and which is able to withstand temperatures as high as 300-400 C., since such temperatures are often obtained during deposition techniques. Thus, a wide range of materials is suitable as substrates, steel being preferred for such purposes.

The present invention is conveniently described in detail by reference to an illustrative example in which a switching contact is fabricated by vacuum evaporation techniques in accordance with the present invention.

Vacuum evaporation is generally conducted at extremely low pressures (see Vacuum Deposition of Thin ing through the filament.

3,125,654 Patented Mar. 17, 1964 Films, L. Holland, S. Wylie and Sons, Inc., New York, 1956). An apparatus similar to that shown in FIG. 1 is conveniently employed for this step. The extent of the vacuum is dictated by consideration of the vapor pressure of the metal to be evaporated. In conventional vacuum evaporation processes, it is generally considered that the vapor pressure of the metal to be evaporated should be at least ten times greater than the pressure to which the system is evacuated. In general, better quality films are obtained at higher vacuums. When using metals with relatively high vapor pressures, the maximum pressure which can be tolerated is that above which the residual gas present interferes with the deposition of a pure metallic film. For the evaporation of a metal such as tin, it has been determined that a pressure of approximately l 10 millimeters of mercury is satisfactory.

Substrate 14 is first rigorously cleaned. Conventional cleaning agents are suitable, the choice of a particular one being dependent upon the composition of the substrate itself. For example, where the substrate consists of copper, an acid cleaning is required to remove oxide films.

Substrate 14 is placed upon platform 13, as shown in FIG. 1, and mask 15 is then suitably positioned. Platform 13 and mask 15 may be fabricated from any refractory material. However, it may be convenient to use a metal, such as aluminum, for such purposes. To obtain a sharply defined deposit it is necessary to have mask 15 bearing against substrate 14 under externally applied pressure.

The usual method of heating the metal to be evaporated is to position it in or in proximity to a filament which may be heated electrically. This is conveniently accomplished by using a tungsten filament in the shape of a coil as shown in FIG. 1 and placing the metal to be evaporated within the coil. The required temperature is obtained by controlling the magnitude of the current flow- Alternatively, a filament of the metal to be evaporated may be used in those instances where the metal has a sufficiently high vapor pressure at temperatures below its melting point.

Vacuum chamber 11 is evacuated to the prescribed pressure and a current is passed through tungsten filament 12 which becomes heated, thereby causing the metal, not

shown in FIG. 1, to evaporate.

For the purpose of this invention, the minimum thickness of the layer deposited on the substrate, i.e., the gold, copper, lead or bismuth layer, is of the order of Angstroms. There is no maximum limit on this thickness, although little advantage is gained by the increase above 2000 Angstroms.

Following the deposition of the underlying layer, the coated substrate is again coated by vacuum evaporation techniques with a thin film of essentially pure tin. The considerations discussed above with respect to the thickness of the underlying layer also apply to the tin film.

Several examples of the present invention are described in detail below. These examples and the illustrations described above are included merely to aid in the understanding of the invention and variations may be made by one skilled in the art without departing from the spirit and scope of this invention.

EXAMPLE I This example describes the preparation of switching contacts by vacuum evaporation of tin on steel substrates. For comparative purposes whisker growth studies Were made by depositing film on glass substrates.

A vacuum deposition apparatus similar to that shown in FIG. 1 was used to deposit tin coatings, the filament being composed of tungsten. The substrates were cleansed by soaking in hot water containing a wetting agent, such as Tergitol Anionic 08 followed by a rinse in running hot water. The substrates were next drained, dried and mounted on platform 13 approximately centimeters from the source and mask 15 placed atop the substrate.

A suitable length of wire of the metal to be deposited was then placed within the tungsten filament and the vacuum chamber evacuated to a pressure of the order of IX 10* millimeters of Hg. Current was caused to flow through the tungsten filament, heating it to incandesccnce and thereby heating the metal wire and causing it to evaporate. The wire was completely evaporated and layers of the desired thickness were produced on the exposed portions of the substrate. Where underlying layers were deposited the procedure was repeated with tin.

The finished contacts were next exposed in the presence of phenol fiber at 95 F. and 30 or 95 percent relative humidity for the purpose of whisker study. Although the phenol fiber is considered as an accelerator of whisker growth it is not essential in making such studies. The results of the whisker study are set forth in Table I below.

The data set forth in Table I clearly indicates that whisker growth is elfectively prevented by deposition of the tin film on copper or gold intermediate layers. Whisker growth was observed under the indicated conditions when tin was deposited on steel, glass or iron.

In order to determine the applicability of other metals to the present invention further whiskerstudies were made by vacuum deposition of a 1000 Angstroms thick underlay on glass substrates and a 500 Angstroms thick outer layer of tin. After heating at 185 C. for two hours and aging for one year the specimens were examined. The results are set forth below in Table II and indicate that copper, gold, lead and bismuth are suitable for the desired purposes.

Although the invention has been described with reference to switching contacts it will be appreciated by those skilled in the art that the present invention may be suitably applied in the fabrication of any electrical components comprising a surface consisting essentially of tin. Further, it will be appreciated by those skilled in the art that the present invention is not restricted to the deposition of films by vacuum evaporation techniques and other methods such as electrodeposition may be suitably employed.

Whatis claimed is: 1. An electrical contacting surface including successively a substrate, a first layer'in intimate contact with said substrate comprising a thin film of one metal selected from the group consisting of gold, lead, copper and hismuth, and a second layer consisting essentially of tin,

said second layer being in intimate contact with said first layer.

2. An electrical contact according to claim 1 wherein said firstlayer is gold.

3. An electrical contact according to claim 1 wherein said first layer is lead.

4. An electrical contact according to claim 1 wherein said first layer is copper.

5. An electrical contact according to claim 1 wherein said first layer is bismuth.

6. A switching element including a pair of cooperating make-and-break contacts, at least one of said contacts comprising a contacting surface including successively a substrate, a first layer in intimate contact with said substrate comprising a thin film of one metal selected from the group consisting of gold, lead, copper and bismuth, and a second layer'consisting essentially of tin, said second layer being in intimate contact with said first layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,177,288 Schellenger Oct. 24, 1939 2,469,878 Hannon et a1 May 10, 1949 

1. AN ELECTRICAL CONTACTING SURFACE INCLUDING SUCCESSIVELY A SUBSTRATE, A FIRST LAYER IN INTIMATE CONTACT WITH SAID SUBSTRATE COMPRISING A THIN FILM OF ONE METAL SELECTED FROM THE GROUP CONSISTING OF GOLD, LEAD, COPPER AND BISMUTH, AND A SECOND LAYER CONSISTING ESSENTIALLY OF TIN, SAID SECOND LAYER BEING IN INTIMATE CONTACT WITH SAID FIRST LAYER. 